The theory is that the Giants have had 50 years of near-biblical bad luck because of the shabby treatment bestowed upon a now missing plaque honoring the late Grant. The plaque was affixed to a memorial for nearly 40 years at the Giants former home in New York’s Polo Grounds.

The bonze plate, honoring Grant’s selfless military service, has not been seen in 50 years.

“It’s a solid curse,” says writer Mickey Bradley, co-author of a new book about baseball paranormalcies.

Bradley points out that since the plaque – which was prominently displayed on the field at the cavernous Polo Grounds – vanished, the Giants have failed to win a World Series title. The club is now gone 54 years since its last ultimate champagne shower.

Grant – no relation to the “Electric Avenue” singer - was a Harvard-educated lawyer and a career .249 hitter, who spent the final three season of his baseball career, from 1913 to 1915, with the Giants.

The legend of the Grant imprecation is detailed in all its eerie glory in the very readable, anecdote-studded “Haunted Baseball: Ghosts, Curses, Legends and Eerie Events” by Bradley and Dan Gordon (The Lyons Press), $14.95.

Other spooky San Francisco angles in the book include several ballplayers interactions with the spirits known to inhabit the St. Francis Hotel.

In 1918, at age 34, Grant volunteered for service in WW I. Months later, Captain Edward Leslie Grant was killed in action in France’s Argonne Forest while leading a heroic rescue of the penned-in “Lost Battalion.”

The memorial – the only one ever placed in the field of play at a big league stadium – was dedicated on Memorial Day, 1921.

The mystery of the missing bronze plate began after the Giants final home game at the Polo Grounds on Sept. 29, 1957 when the club – having announced plans to relocate to S.F. weeks earlier – was literally chased off the field by jilted New Yorkers.

That afternoon, marauding ticket holders dug up home plate, yanked box seats from their moorings, and pilfered ball caps from the likes of Marv Grissom, Foster Castleman and Windy McCall – by the way, isn’t it a shame he didn’t make the trek west with the club? – as the players made a mad dash to their clubhouse located beyond center field.

As they scrambled up the stairs to the safety of their locker room, the ball players passed the crux of their possible lingering torment – the Grant plaque sitting atop its five-foot tall anchor.

Where that plaque has gone since that day is unknown.

Multiple theories abound. Initially it was believed that a gang of young toughs loosened the plaque from its monument with a hammer or some other type of prying device. (Let’s see got my glove, pencil and scorecard, whoops, almost forgot the crow bar!)

Another story had the plaque in the possession of a NYC cop. Then there’s the theory that some besotted sporting scribes spiriting away the heavy souvenir under the cover of night and stashing it in Toots Schor’s place.

In recent years the Giants quietly even searched their own storage areas – coming up with pieces dedicated to Mel Ott and Christy Mathewson, but not the Grant plaque.

“The bottom line is no one knows where it is. No one has seen it,” says Bradley, who estimates the plaque’s value at $20,000.

Bradley and Gordon write with irony-free plausibility of cataclysmic events that have occurred to the Giants, since the plaque went missing, including two gut-wrenching finishes to the 1962 and 2002 World Series and the deadly earthquake that interrupted San Francisco’s 1989 fall classics appearance.

Over the past few seasons, the legion of Grant’s plaque jinx has picked up some steam that club officials may have even bought into. In 2006, the Giants even discreetly acknowledged the possibility of a curse by ordering an exact remake of the plaque and positioning it on the side of the southwest corner AT&T Park near the Lefty O’Doul bridge.

Mysteriously, Bradley says, the Giants – who routinely issue press releases at the drop of a hat, or even if a player drops his hat – have never publicly acknowledged the installation of the replacement plaque. You won’t find mentioned of it in the club’s media guide or on the Giants official web site.

“I can only think that they don’t want to call too much attention to the idea of a curse,” says Bradley. “It is a little odd that they installed it in such a quiet low key kind of way. Normally it would be the type of thing you would want to publicize.”

If the installation of the new plaque was supposed to spark the club’s rise in the standings it hasn’t quite kicked in yet – since its placement the Giants have endured the worst two year stretch in years.

It’s clear something more has to be done. It’s unlikely the plaque will ever be returned to center field (unless you enjoy a side of impaled outfielder with your Cha-Cha Bowl).

But what about making a peace offering to the man in question? The heroic solider/second baseman is buried not far from where he fell on October 5, 1918. Grant’s tomb is located at the Meuse-Argonne American Cemetery in Romagne, France (Plot A, Row 2, Grave 24).

It just might take a continent of Giants fans to make the journey there to honor his memory and maybe leave a peace offering. Say, a fine Cabernet Sauvignon, a ripe round of Camembert and crusty leaf of San Francisco French bread.

Call me if you make the trip. If we go during baseball season, we can take a break from the “Plague of the Plaque.” Even that cheese has to smell better than the Giants recent fortunes.